1. Field of the Invention
The present invention relates to an apparatus and methods for removing acetylene from a cracked gas mixture; and in particular, to an apparatus and methods for removal of acetylene from a cracked gas mixture using a palladium containing catalyst.
2. Background Art
Cracking is a chemical process in which high temperature is applied to break apart and convert heavier hydrocarbons in lighter hydrocarbons. Various hydrocarbons and hydrocarbon mixtures are cracked to prepare a number of lower alkenes which may be used in a multitude of chemical manufacturing processes. In particular, cracking processes are used in the production of ethylene, an important precursor to many types of plastics. After cracking, a mixture of hydrocarbon compounds that must be subsequently purified is the result. The removal of acetylene from ethylene producing processes is of particular concern because of the difficulty in removing this compound.
Currently, a number of processes exist for the removal of acetylene from ethylene producing processes. Such processes include, front-end de-propanizer acetylene removal units (“ARU”), front-end de-ethanizer ARU, and tail-end ARU. Front-end de-propanizer ARU typically employ a Pd catalyst acetylene hydrogenation system located on the front-end (of distillation train) de-propanizer overhead stream. This ARU hydrogenates all of the acetylene and most of the methyl acetylene/propadiene (“MAPD”) on a propane and lighter stream. In processes utilizing Front-End De-Ethanizer ARU, a Pd catalyst acetylene hydrogenation system located on the front-end (of distillation train) de-ethanizer overhead stream is typically used. This ARU hydrogenates all of the acetylene, but not MAPD on an ethane and lighter stream. If a plant of this type is maximizing propylene recovery, a separate MAPD hydrogenation reactor system and possibly a green oil removal system on the C3 product stream may be required.
The tail-end acetylene removal system usually employs a Pd catalyst acetylene hydrogenation system located on the de-ethanizer overhead stream, where the de-ethanizer is located after the de-methanizer in the distillation train. This ARU hydrogenates all of the acetylene, but no MAPD, on a stream primarily composed of ethane and ethylene. Moreover, tail-end ARU systems normally require a green oil removal system on the C2 product stream. Furthermore, if a plant of this type is maximizing propylene recovery, a separate MAPD hydrogenation reactor system and possibly a green oil removal system on the C3 product stream may be required.
Each of the acetylene removal processes described above requires the utilization of expensive processing equipment. Moreover, each of the prior art acetylene removal processes utilize a catalyst for the selective reduction of acetylene. Experience has taught that such catalysts are prone to runaway reactions that may potentially cause a dangerous temperature rise or excursion. Start up of the acetylene removal units, especially with fresh catalyst is the time when these reactors are most susceptible to runaway reactions.
Accordingly, there exists a need in the prior art for an improved methods and apparatus of removing acetylene from a cracked gas mixture with reduced capital equipment costs. Specifically, there is a need for an improved method and apparatus for starting up such acetylene removal units and in the case of a thermal runaway for shutting down acetylene removal units.